Using bovine pulmonary artery endothelium cells (APAE), we determined some of the effects of PDT on subcellular areas of the APAE cells. Three hour treatment of 50 mg/ml ALA permitted optimal detection of fluorescence within the cell. Bright fluorescence was confined primarily to the mitochondria. There were no adverse effects in controls (with no ALA) while receiving five minutes of exposure, however, some of the cells with ALA appeared sick or lysed within 24 hours of exposure. Both cells with and without ALA were more sensitive to irradiation of the nuclei than irradiation of the cytoplasm. Almost all cells with ALA that had irradiation of the nuclei lysed with 24 hours of exposure.